1. Field of the Invention
This invention relates to the millicode execution of complex floating point operations in a computer system conforming to the IBM z/Architecture Principles of Operations, and particularly to passing operand data from program floating point registers to millicode working registers, and passing results from the millicode working registers back to program floating point registers using register indirect accesses.
2. Description of Background
Complex instructions are executed under millicode control when it is not cost effective to implement its function in hardware alone. The function of the complex instruction is programmed in a routine that uses sequences of simpler hardware instructions.
One of the disadvantages to executing complex instructions using millicode routines is determining what the source and destination operands are for the operation. It is not known what program registers an instruction will use for any particular user program, so it is not possible to write millicode routines that move operand data directly between the program floating point registers and the millicode routine working registers. Instead, when control passes from a program to a millicode routine for the purposes of executing a complex instruction, the original instruction text is parsed to determine what program registers are the source for the instruction. Next an instruction is created that will copy the data to the millicode working registers. This process of copying program register data can be very costly for performance because it requires modifying the instruction stream. Likewise, writing the result from a millicode working register back to the program floating point register (FPR) requires the same process of parsing the instruction text, creating an instruction and forcing that instruction into the instruction stream where it is executed.